Why did Jesus wait till he was a grown man to be baptized? Matthew 3:13,“Then came Jesus from Galilee to Jordan unto John, to be baptized of him.”. . …
Because He wasn’t undergoing a Christian baptism (how could he?!

) ; He submitted to the baptism performed by John as a sign of His entry into His public life (and, as the Fathers teach, His entry into the water sanctified it as the sign of Christian baptism).
The baptism performed by John is that it is “a unique baptism in the desert in view of the repentance and pardon (Mk 1:4p). . .The baptism of John set up only a provisional economy: it is a baptism of water which is preparatory to the Messianic baptism in the Holy Spirit and fire,” Xavier Leon-Dufour, Dictionary of Biblical Theology. It was an external ritual signifying an effort of conversion of the baptized (much as many Protestant denominations look upon baptism today).
The baptism of John was the “mikveh”, a ritualistic bath which was a common “rite” in Jewish worship. However, in the case of the baptism of John, he was applying the mikveh in a particular sense: a sense of conversion TO Judaism. When one becomes a Jew (even today, in Orthodox Judaism), one has his head shaved and his fingernails and toenails closely cut - as if one is a baby again; and one is immersed into the mikveh (the ritualistic bath) so as to emerge and be “reborn” as a Jew. This is exactly what John was doing as the precursor to the Messiah.
For 2000 years God has asked the people of Israel to keep faith with His Commandments - and for 2000 years the Israelites had failed to do this. Yet, now St. John comes along preaching a baptism of repentance to prepare for the Messiah. Here, what St. John is saying is: "Okay, everybody! Come on back! Become REAL Jews and truly commit yourself to the Jewish Covenant to which you have been unfaithful. THIS is what is meant by the “baptism of John” - a baptism into Judaism, into the Jewish Covenant - in preparation for the Messiah’s coming.
However, Jesus’ Baptism (the Baptism He instructs His Apostles to perform in the Name of the Father and of the Son and of the Holy Spirit) will do something greater. It will be the Baptism into a NEW Covenant - a Baptism into His Church. The Greek work for “Church” (Ekklesia) means “those who are called out” - that is, out of Judaism, out of the old Covenant of law and into the New Covenant of love. Thus the Church will be that remnant of Israel which, along with the Gentiles, will accept Jesus as their Messiah and King. John’s baptism was not enough to accomplish this. While John’s baptism made one fit to be a Jew (one faithful to the Covenant), it did not make one “born again” into the new Covenant in Christ - just as Christ tells Nicodemus in John 3. So a new baptism was necessary.
John’s baptism, therefore, was NOT for the removal of Original Sin and being born into the New Covenant; rather it was a preparation for the pouring out of the Holy Spirit through the Death and Resurrection of Our Savior. It is the Christian baptism by water and the Spirit into His Death and Resurrection that accomplishes in the soul (1 Cor. 1:13; Romans 6:3 & 9) what it signifies - a rebirth into Christ, the Sinless One, who signs us with His Cross and the power of His Resurrection. It is the new and greater “circumcision” that now joins together Jew and Gentile, male and female, adult and babe into the Familial Covenant of Christ.